This project concerns the chmistry of a group of trimethylenemethane (TMM) derivatives, the 2-alkylidenecyclopenta-1,3-diyls. The objectives are to delineate the role of the spin states, triplet and singlet, in the chemistry of these species and to elucidate the behavior of the closely related highly strained hydrocarbons belonging to the bicyclo(3.1.0)hex-1-ene and 5-methylenebicyclo(2.1.0) pentane series.